About Me

Sunday, November 14, 2010

Testing Tenergy 9V NiMH batteries (discharge curve)

I recently bought some 250mAh 9V NiMH batteries and a charger made by Tenergy. I plan to use these batteries in a Shure PG58 wireless mic. I first tested the current draw by the mic from a bench power supply, and found that is was very close to 50mA from 6.5V to 10V input voltage, and did not fluctuate based on sound input to the microphone.

I connected a DMM in parallel with the battery that is plugged directly into the microphone.

Test 1 = battery charged until the charger LED changed to green, then the battery was removed and left on the shelf for 2 days before testing in the microphone

Test 2 = battery charged for two days (fast charge, then automatic trickle), removed from charger and immediately tested in microphone.

I am slightly disappointed since the battery is supposed to be 250mAh, and I am only discharging it at 50mA ( .2C), so I should get nearly full capacity, but instead only get 200mAh effective capacity when the battery is fresh out of the charger -- 150mAh capacity after a couple days . It will probably be good enough for my application, but this is more evidence that battery manufacturers are inflating their ratings.

I popped open the charger and found a circuit board with a single IC (part number is obfuscated -- possibly not intentionally), lots of SMD transistors and assorted passives.

Here's an oscilloscope screenshot that shows the charging current and voltage behavior. This battery is almost completely discharged. The charger maintains a constant current of 105mA, with a 167ms gap every 1.7 s where no current is put into the battery. Presumably, this is so the charger can monitor the unloaded battery voltage. As you can see, the battery voltage drops from 9.6V to 9.5V in the 167ms gap. As the battery charges, the charge voltage (10 V in this screenshot) rises to accommodate the battery's increasing state of charge.

6 comments:

Ben, if you recall the tests we did with the box of various Tenergy batteries I bought a few years ago-- the ratings are outright lies. That's just how these cheap Chinese batteries work. I recall purchasing "10000mAh" D-cells and they had maybe a fifth of that capacity at 0.5C... and it got exponentially worse with higher discharge (I think we tested to 3C). You have to buy batteries from "real" manufacturers (Sony, Sanyo, Panasonic, etc) if you want to come close to the performance on the label.

Peter, I had forgotten that your batteries were made by Tenergy. Wow, at only .5C they were already struggling? When I started hunting for these 9V batteries, I figured a discharge of only .2C would mean I could get full capacity no problem. Oh well, we will have to change batteries in the microphones between sets, but this should be pretty easy.

I purchased four Tenergy 9 volt 250 ma batteries for a sure wireless mic and have been dissappointed in their performance. Two of the batteries will no longer charge. Each battery has been charged about 20 times.

hi ben, i was planning to use one of these 9V battery. Only concern I have is charging. I have a system which generates very low power around 150mA at 12V which I was planning to charge my batteries with. So a high end branded charger is not a possibility. So i wanted to know the charging intricacies of these batteries? Firstly i presume since these are single pack hence doesnt require balancing. Am i right in that?

And can you charge them at a constant power or do u have to vary the power depending upon the voltage across the battery?